The invention relates to a method and apparatus for reducing uranium and/or plutonium oxides to the metal. More specifically, this invention relates to a method and apparatus for electrolytically reducing uranium and/or plutonium oxides to a high-purity metal product.
High-purity uranium metal is the required feed material for enrichment of .sup.235 U by the Atomic Vapor Laser Isotope Separations (AVLIS) process. A supply of high-purity uranium and plutonium metals is also important in the preparation of nuclear reactor fuel rods.
Uranium metal can be prepared from uranium oxides by a variety of methods. The commercial method for the production of uranium metal consists of converting the uranium oxides to UF.sub.4, and then reducing the UF.sub.4 to metal with calcium by thermochemical reduction. The uranium oxides can also be reduced by electrolytic methods using a molten salt as the electrolyte. Both processes are carried out at high temperatures (1200.degree. C. or higher) and the product is collected as liquid metal. Chemical reductions involving a liquid mixture such as a zinc-magnesium alloy as the reductant at temperatures of 700.degree.-800.degree. C. have been demonstrated in the laboratory. In this process, the uranium metal product is recovered by vaporizing off the solvent metals in a distillation or retorting operation.
Electrolytic processes for the reduction of uranium oxides to uranium metal have advantages over chemical reduction methods. For example, they do not produce the by-product wastes which must be disposed of that chemical methods do, they generally operate at a lower temperature which reduces the problem of finding suitable materials for the process equipment, they can be operated in such a way as to minimize recycle streams in the process, and they can produce a high-purity product.
Several electrolytic processes have been developed for the production of uranium metal from various uranium salts. U.S. Pat. No 3,330,742, describes a method for electrolytically reducing uranium hexafluoride to uranium metal in a fused salt. The UF.sub.6 gas is contacted with a graphite anode and an electrolyte containing UF.sub.4 and an alkaline earth fluoride at about 1150.degree. C. Carbon tetrafluoride gas is evolved at the anode while the molten uranium metal forms as a pool at the cathode. Another process is based on the concept of the Hall Process for producing molten aluminum from its oxides in that the oxygen reacts with carbon at the anode to form oxides of carbon. The process was found to be operable and uranium metal was obtained However, the metal product suffered from poor coalescence caused by oxide contamination and low product yields. Other processes have incorporated the uranium oxides into a consumable oxide-carbon anode. Still other processes have tried feeding the uranium oxide directly into the cell, utilizing the crucible as the cathode.
All the processes for the electrolytic reduction of uranium oxides suffer from difficulties which are related to the low solubility of the uranium oxides in the electrolyte. The high operating temperatures of the prior art processes, not only increase operating costs, but produce a molten uranium metal product which is contaminated with insoluble uranium oxides from the electrolyte. These impurities produce poorly coalesced uranium metal particles requiring extensive purification and processing before the metal product can be further utilized.